Radiation detectors equipped with a specially adapted filter arrangement, such as for example interference filters or monochromators, are often used to detect radiation with a defined spectral sensitivity distribution that has a maximum at a defined wavelength λ0. Such detectors are characterized by a very good match with the defined spectral sensitivity distribution, but are usually comparatively labor- and cost-intensive to operate and produce. In addition, they frequently have high spatial requirements, making them of limited or no use in small-space applications.
If the defined spectral sensitivity distribution is that of the human eye, then a silicon photodiode is often used to detect incident radiation with this sensitivity.
The sensitivity of a photodiode depends, among other things, on the wavelengths of the incident radiation. For wavelengths above a cutoff wavelength the sensitivity is at least nearly zero, since for incident radiation in this wavelength range the energy gap of the functional material of the diode—Si, for example—is greater than the energy of the incident radiation, and the latter is therefore insufficient to generate electron/hole pairs. On the other hand, the sensitivity decreases in the range of diminishing wavelengths, since as the wavelength declines the electron/hole pairs that are generated, for example by surface recombination, progressively stop contributing to the photocurrent. In the intermediate range, the sensitivity of the diode presents a maximum which in the case of a silicon photodiode is located at about 800 nm.
To use such a silicon photodiode as a detector with the spectral sensitivity distribution of the bright-adapted human eye, which has a sensitivity maximum at about 555 nm, requires extra expenditure, since the wavelengths of the sensitivity maxima differ greatly from each other and the two spectral sensitivity distributions are therefore relatively poorly matched. The matching of detector sensitivity to the sensitivity distribution of the human eye can be improved through the use of multiple complex filters. The sum of these filters yields the sensitivity of the human eye.